Slightly soluble inorganic salts, for example calcium carbonate, calcium sulfate, and barium sulfate, often precipitate from waters produced along with oil and gas. Such precipitation occurs because conditions that affect solubility, such as temperature and pressure, change in, or near, producing wellbores. Additionally, incompatible waters can mix with formation waters in or near the wellbore. Precipitates of slightly soluble salts are often called scale. Scale build-up occurs both within the near wellbore formation and within the wellbore equipment. Scale build-up within the formation decreases permeability and impairs well productivity. Scale build-up within wellbore equipment shortens equipment useful life and can plug production tubing.
To minimize scaling, scale inhibitors can be placed within the formation by "squeeze" methods. Squeezing involves injecting the inhibitor and, usually, overflushing the treated zone with water or brine. The well is then placed back into production. Inhibitor in the produced waters protects the wellbore and downhole equipment from scale build-up.
Squeezing inhibitor into the formation is a convenient way to apply an inhibitor. Continuous injection is not required, and the formation in the vicinity of the wellbore along with the wellbore equipment is protected. Most common scale inhibitors are effective in concentrations greater than about 1 to 100 parts per million. In a normal squeeze placement, it is difficult to control the concentration of inhibitor returning in produced brines. The inhibitors tend to be produced quickly, with concentrations quickly tailing off to ineffective amounts. This results in frequent shutdowns of production for squeezing operations, and also in excessive chemical costs.
Various attempts to control and delay inhibitor production have been only partially successful. For example, U.S. Pat. No. 3,483,925 discloses a method to provide a more gradual return of inhibitor by injection of the inhibitor with polymer-thickened slugs of solution. Injection of such thickened solutions would inhibit production of all fluids from the wellbore, and is therefore not a desirable method to provide a more gradual return of the inhibitor when production from the wellbore is desired.
U.S. Pat. Nos. 3,633,672 and 3,704,750 disclose inhibitors which are only slightly soluble in neutral or basic solutions and soluble in acidic solutions. These inhibitors are squeezed into the formation in an acidic solution, and then dilution with formation brine and reaction with reservoir rock raises the solution pH and causes precipitation of the inhibitor. The inhibitor is then produced gradually due to a slight solubility in the brine. This method solves many of the problems with squeeze placement of scale inhibitors, but due to plug flow into the formation and then back with production, much of the inhibitor is not precipitated into the formation within a reasonable amount of time. Further, placement of the precipitate is poorly controlled and not predictable.
U.S. Pat. No. 4,357,248 discloses an improvement to processes such as those disclosed in U.S. Pat. Nos. 3,633,672 and 3,704,750. Patent '248 discloses injection of a scale inhibitor in a form which is soluble in a high pH aqueous solution and insoluble in neutral or low pH solutions. The solution further comprises a compound that hydrolyzes to form acidic compounds at a rate which causes the inhibitor to precipitate in the formation without the need for mixing with formation brines or reaction with formation rocks. This method, in spite of its advantages, has not been widely practiced because of difficulty in handling the useful pH lowering materials.
U.S. Pat. No. 5,141,655 and application Ser. No. 829,185 disclose an alternative to injecting pH lowering compounds to cause scale inhibitors to precipitate. These patents disclose the inclusion of pH raising compounds and starting with an initial scale inhibitor solution that is of a low pH. Typical scale inhibitors are soluble at low and high pH's, and are less soluble over a pH range of about 3 to 6. Although the low pH inhibitor solutions are easier to handle, they must still be blended with the pH adjusting compounds at the time of injection.
It is therefore an object of the present invention to provide a method to place a scale inhibitor into a subterranean formation and composition suitable for such a method wherein the formation comprises CaSO.sub.4, the scale inhibitor is soluble in the solution prior to contact with the subterranean formation, and the solution does not require mixing at the wellhead prior to injection. It is a further object to provide such a method and composition wherein reactive chemicals are not required for wellhead preparation of such a composition.